This is about a cluster of content that is created from a main theme. You create a variety of topics as subcategories and link to the page with the main theme. This means you have one umbrella topic and many other pieces under it so that your readers who are interested in that topic stay on your site for a longer period of time till they get all the information. It also gives you the opportunity to place hyperlinks of these pieces among themselves as related topics. 

Topic clusters are a great way to optimize the content on your website because you are able to create a lot of content as a voice of authority that is relevant to the user. It is kind of like the opposite of one long-format piece. 

When you create five or six pieces on the same subject with different angles, you can also give the user the impression that you are covering a range of subjects and convince them that you are an authority on the subject.